1. Field of the Invention
The present invention relates to a fabrication method for a thin film transistor, and more particularly to a fabrication method utilizing laser ablation technology to simplify the manufacturing of thin film transistors.
2. Description of the Related Art
FIG. 1a to FIG. 1e illustrate a conventional process for manufacturing a thin film transistor in liquid crystal display using five photomasks. In FIG. 1a, a first conductive layer is formed on a substrate 100 by sputtering. Then, the first conductive layer is patterned to form a gate electrode 120 on the substrate 100 by a first photolithography and etching process beyond a predetermined thin film transistor area T.
In FIG. 1b, an insulating film 140, a semiconductor film 160, and an ohmic contact layer 180 are sequentially formed on the gate electrode 120. The semiconductor layer 160 and the ohmic contact layer 180 are patterned by a second photolithography and etching process to remove portions beyond the predetermined thin film transistor area T.
In FIG. 1c, a second conductive layer is formed on the substrate 100 by sputtering. The second conductive layer and the ohmic contact layer 180 underlying the second conductive layer are patterned by a third photolithography and etching process to form a drain electrode 200a and a source electrode 200b on the predetermined TFT area T. A portion of the semiconductor layer 160 exposed by an opening serves as a channel region. More specifically, the semiconductor layer 160 is exposed by an opening located between the drain electrode 200a and source electrode 200b. 
In FIG. 1d, a passivation layer 220 covers the substrate 100 and the island shaped structure on the thin film transistor area T. The passivation layer 220 is then patterned by a fourth photolithography and etching process to form a contact hole 240 therein, thereby exposing a portion of the source electrode 200b. 
In FIG. 1e, a pixel electrode 260 is formed on a portion of the passivation layer 220 and patterned by a fifth photolithography and etching process. The pixel electrode 260 is electrically connected to the source electrode 200b through the connect hole 240.
As mentioned above, the process for manufacturing the thin film transistor typically requires the use of five photomasks. Since manufacturing costs are greatly dependent upon the total number of photomasks used, it is a general object in the art to save the manufacturing costs by reducing the number of photomasks. It has been proposed in other fields, to use a laser ablation process to eliminate the need for photomasks to simplify the manufacturing steps. For example, in U.S. Patent Publication No. 20050064648, irradiation of the laser beam is performed on a photothermal converting layer to transfer heat and therefore sublimate a portion of a sublimation layer thereon. U.S. Patent Publication No. 20050258478 uses laser ablation to form a groove pattern in a semiconductor film.